1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunctional machine comprising two or more of these in combination, and especially to a developing device used for the image forming apparatus.
2. Description of Related Art
Image forming apparatuses such as copying machines, printers, facsimile machines, or multifunctional machines comprising two or more of these in combination charge the surface of an electrostatic latent image carrier by a charging device, subject the charged region to image exposure from an exposure device to form an electrostatic latent image, develop the electrostatic latent image by a developing device to form a toner image, and transfer the toner image onto a receiver object.
Herein, the “receiver object” is normally recording mediums such as recording papers in monochrome image forming apparatuses. Meanwhile, in full-color image forming apparatus and the like, when an intermediate transfer member onto which a toner image on an electrostatic latent image carrier is primarily transferred is employed, both the intermediate transfer member and the recording medium onto which the toner image is secondarily transferred from the intermediate transfer member are considered as receiver objects.
Known typical developing devices in such an image forming apparatus include those which use dry developers, and those which use liquid developers. Today, developing devices which use dry developers are common. Known developing devices which use dry developers include those which use so-called one component developers which are mainly composed of a toner, and those which use so-called two-component developer containing toner and carrier particles.
In general, developing devices which use a two-component developer comprise a fixedly disposed magnet body, and a development sleeve which is rotatably fitted onto the magnet body, forms and retains developer brushes comprising a developer containing a toner and magnetic carrier particles on the surface of the development sleeve by the magnetic force of the magnet body, transfers the developer brushes to a development region in which an electrostatic latent image formed on the surface of the electrostatic latent image carrier which is rotationally driven is developed, and the developer brushes are brought into contact with the surface of the electrostatic latent image carrier to develop the electrostatic latent image.
FIG. 19(A) shows an example of a magnet body MR and a development sleeve DS fitted onto the same in such a developing device. The magnet body MR has, as shown in FIG. 19(A) as an example, a magnetic pole group comprising N-poles and S-poles arranged annularly. Among these magnetic poles, the magnetic pole facing a development region Da′ in which an electrostatic latent image is developed on an electrostatic latent image carrier (photosensitive member in the example illustrated) PC is a development pole Dp′ which is mainly involved in development of the electrostatic latent image. In the example shown in FIG. 19(A), one N-pole facing the development region Da′ serves as the development pole Dp′
At such a development pole Dp′ composed of a single magnetic pole, the distribution of magnetic force is normally substantially symmetrical with respect to a center Dpc of the development pole Dp′ as shown in FIG. 19(B) as an example.
FIG. 20(A) schematically shows examples (1) to (6) of the magnetic brushes of a developer formed on the development sleeve DS in the development region by such a development pole Dp′. In the example shown in FIG. 20(A), the direction of movement of the surface of the development sleeve DS in the development region, therefore the direction of transfer of the developer, is d1, and the direction of movement of the surface of the electrostatic latent image carrier PC in the development region is d2, which is opposite to d1.
The magnetic brushes (1) to (6) of the developer comprises carrier chains which are composed of the magnetic carrier particles Cp formed on the surface of the development sleeve DS by the magnetic pole(s) such as the development pole, and a toner t deposited to these.
As shown in FIG. 20(A), the magnetic brushes by the development pole Dp′ start to be generated slightly upstream of the development region in the direction of transfer of the developer d1 in which the magnetic flux density in the direction of the tangent of the surface of the development sleeve DS is high, and are transferred downstream with the shape of the brushes maintained.
Described in further details, a magnetic brush starts to rise up in a portion adjacent downstream of the middle position between the development pole Dp′ and the adjacent upstream magnetic pole (not illustrated). As shown in FIG. 20(A), a long magnetic brush (1) then rises up in the vicinity of an upstream end portion of the development region Da′. A magnetic brush (2) whose forward end portion is in contact with and bent by the image carrier PC downstream of the brush (1) starts to rub the surface of the image carrier PC. On a further downstream side, the bent portions of the brushes (2) become magnetic brushes (3), (4) and (5) which have been reduced in length but increased in thickness in a manner of being folded, and rub the surface of the image carrier PC. A magnetic brush (6) which remains short and thick is detached from the image carrier PC near the downstream end of the development region Da′.
In development of the electrostatic latent image by the magnetic brushes formed in such a manner, as schematically shown in FIG. 20(B), the forward end portions of long magnetic brushes (21), (22) as the magnetic brush (1) of FIG. 20(A) are likely to act in a manner of scraping off the toner t deposited to the electrostatic latent image. When a large amount of the toner t deposited to the electrostatic latent image is scraped off, image deficiency occurs and image density is lowered, whereby the image quality is lowered.
As schematically shown in FIG. 20(C), in a thick and short magnetic brush (40) such as the magnetic brushes (3) to (6) in FIG. 20(A), the degree of freedom of the toner t in the brushes is likely to be lost, and its contribution to the development is likely to be difficult. When the degree of freedom of the toner t is lost and the amount of the toner deposited to the electrostatic latent image is reduced, development efficiency is lowered, image deficiency occurs, and image density is lowered, which lowers the image quality.
Generally speaking, when a developing device in which the development pole is constituted by a single pole is used by being mounted on an image forming apparatus with high processing speed (the rate of image formation process), there is a known tendency of lowered development performance. In particular, there is a tendency that the carrier particles are deteriorated by a change in their surface shapes in image formation, deposition of toner resin components and for other reasons, and accordingly the amount of the developer transferred by the magnet body and development sleeve is reduced, and the density of the toner image which is developed and formed is reduced.
Japanese unexamined Patent Publication No. H5-72902 (JP, 05-72902, A) describes that in order to solve such a problem, the development pole facing the development region is rendered a same-polarity development pole which is magnetized so as to have adjacent poles having the same polarity.
FIG. 21(A) shows an example of the distribution of magnetic force by a so-called same-polarity development pole in which adjacent poles having the same polarity (N-poles or S-poles) have been magnetized. FIG. 21(B) shows an example of the magnetic flux density of a portion containing the development pole of a magnet body having a same-polarity development pole. In FIG. 21(B), Br is the magnetic flux density in the normal direction with respect to the surface of the development sleeve, and BO is the magnetic flux density in the direction of a tangent of the surface of the development sleeve. f shows the magnetic attraction force (f=Br+Bθ).
In FIG. 21(B), the central angular position on the magnet body shown on the horizontal axis is a position corresponding to the angular amount (referring to FIG. 2, the central angle amount of revolution in the counterclockwise direction in FIG. 2), with reference to the central position of a so-called D cut face (D cut face Dc as shown in FIG. 2 described later) for positioning and fixing the magnet body formed at an end portion of the shaft of the magnet body. In the example shown in FIG. 21(B), the angular position of the center of the development pole is a position mp which is 290 degrees from the reference position in the counterclockwise direction.
As can be seen from FIG. 21(B), when the same-polarity development pole is employed, the magnetic force f is abruptly reduced between the adjacent same-polarity portions, which lowers the magnetic attraction force and causes the disturbance of the developer. Therefore, the toner is rendered more mobile in this portion proportionally, and the developability of the electrostatic latent image is improved proportionally. Normally, the generation of stripe-like image noise which may occur when there is clogging in a controlling member (brush height controlling member) which controls the height of the developer brushes provided upstream of the development region in the direction of rotation of the development sleeve is advantageously suppressed by the disturbance of the developer resulting from a decrease in the magnetic constraining force. Furthermore, the contact resistance of the developer to the electrostatic latent image carrier in the development region is reduced because of a repulsive magnetic field caused by the adjacent magnetized portions having the same polarity, and therefore the disturbance in the image is proportionally suppressed, and the image quality is advantageously improved.
However, in a developing device which employs the same-polarity development pole, the magnetic force is abruptly lowered in the central portion between adjacent magnetized portions having the same polarity. Therefore, the amount of toner scattering is large in the portion of the development region corresponding to the central portion, and the image is thus likely to have fogging.
Setting the central portion between the same-polarity magnetized portions in a position where the developer leaves the electrostatic latent image carrier in order to suppress the fogging phenomenon by quickly removing the central portion between the same-polarity magnetized portions away from the electrostatic latent image carrier causes the carrier particles to be easily deposited to the electrostatic latent image carrier due to the disturbance of the developer resulting from a decrease in the magnetic force in the central portion.